1. Technical Field
The present disclosure relates to acoustic devices and, particularly, to a thermoacoustic device.
2. Description of Related Art
An acoustic device generally includes a signal device and a loudspeaker. The signal device provides electrical signals to the loudspeaker. The loudspeaker receives the electrical signals and then transforms them into sounds audible to humans.
There are different types of loudspeakers that can be categorized according to their working principles, such as electro-dynamic loudspeakers, electromagnetic loudspeakers, electrostatic loudspeakers and piezoelectric loudspeakers. However, the various types ultimately use mechanical vibration to produce sound waves, in other words they all achieve “electro-mechanical-acoustic” conversion. Among the various types, the electro-dynamic loudspeakers are most widely used. However, the electro-dynamic loudspeakers are dependent on magnetic fields and often weighty magnets. The structures of the electric-dynamic loudspeakers are complicated. The magnet of the electric-dynamic loudspeakers may interfere or even damage other electrical devices near the loudspeakers.
Thermoacoustic effect is a conversion of heat into acoustic signals. The thermoacoustic effect is distinct from the mechanism of the conventional loudspeaker, in which the pressure waves are created by the mechanical movement of the diaphragm. When signals are supplied to a thermoacoustic element, heat is produced in the thermoacoustic element according to the variations of the signal and/or signal strength. The heat propagates into surrounding medium. The heating of the medium causes thermal expansion and produces pressure waves in the surrounding medium, resulting in sound wave generation. Such an acoustic effect induced by temperature waves is commonly called “the thermoacoustic effect”.
Carbon nanotubes (CNT) are a novel carbonaceous material having extremely small size and extremely large specific surface area. Carbon nanotubes have received a great deal of interest since the early 1990s, and have interesting and potentially useful electrical and mechanical properties, and have been widely used in a plurality of fields. Xiao et al. discloses an thermoacoustic device with simpler structure and smaller size, working without the magnet in an article of “Flexible, Stretchable, Transparent Carbon Nanotube Thin Film Loudspeakers”, Xiao et al., Nano Letters, Vol. 8 (12), 4539-4545 (2008). The thermoacoustic device includes a carbon nanotube film loudspeaker. The carbon nanotube film used in the thermoacoustic device has a large specific surface area, and extremely small heat capacity per unit area that make the sound wave generator emit sound audible to humans. Accordingly, the thermoacoustic device adopted the carbon nanotube film has a potential to be actually used instead of the loudspeakers in prior art.
However, the drawn carbon nanotube film is formed by drawing from a carbon nanotube array. The size of a single drawn carbon nanotube film is limited by the size of the carbon nanotube array. Thus, the size of the loudspeaker is difficult to be enlarged. Further, the carbon nanotube film drawn from the carbon nanotube array is very thin and weak. Therefore, when the large single carbon nanotube film is used, it is hard to avoid damage of the carbon nanotube film. Therefore, a large loudspeaker is difficult to be achieved.
What is needed, therefore, is to provide a well protected thermoacoustic device with a desired large size.